Info about "high efficiency" furnace

[gearguru]

I hope that I am in right forum.
Lately I had the estimates done for my small houses (upstate NY) the central air and heating. All companies are trying to sell me the high efficiency furnace. They say, that the lower gas consumption will pay quickly for the price of the furnace. My question is - does it make sense to have more complicated furnace with an extra fan and a hole in the side of my house? Are these furnaces really that good? Also - does somebody have an info about reliability of different products (Kenmore, Carrier, Trane, other)?
Any information will help!
Thanks!
gearguru

 

[KenRad]

gearguru,

It would be a good idea to do a rough payoff calculation yourself. You need to get the efficiency numbers for the furnace you’re replacing, and compare it to the efficiency of the new unit. If the old unit is less than 12 years old, it’s probably an 80% AFUE (annual fuel utilization efficiency); 20 years or older, could be as low as 60%. The newer high efficiency units that your contractors are trying to sell will be 90-95%. I would guess that in upstate New York your furnace is the biggest chunk of your gas bill, so you’ll probably come up with a fairly good payoff number. Make sure that the new unit is not oversized, or some of the efficiency gain will be lost in excessive cycling.

Concerning the extra fan, I suppose you’re talking about the induced draft blower. This is a small, fractional horsepower direct drive fan, that should require no maintenance (at most, a light cleaning every few years). And the hole (or holes) in the side of the house for combustion air should only be 2” or so in diameter; not very noticeable. Any new unit you install will be more complex than what you’re used to, with microprocessors and more sensors. But these technologies have been around for a while, and it’s been my experience that these units are very reliable (I can only speak for Trane and Carrier). Some of the controllers will even give you a specific error code when there’s a problem, to aid in troubleshooting.

 

[gearguru]

Thanks, KenRad!
I appreciate your help. I'll do my math.
Do you know where can I find the heat-loss calculations for checking the capacity of my furnace?
I also have one more question about the "hole in the wall": Isn't there a problem with the condensig water from combustion products around these "holes"?
Thanks again - and have a nice weekend!
gearguru

 

[KenRad]

gearguru,

The holes in the wall that the contractor is talking about on the higher efficiency units are for PVC pipe. Some units pull combustion air from the room where your furnace sits, and discharge the relatively cool byproducts through the wall using regular schedule 40 PVC pipe (ie, one hole through the wall). Some units are designated as "sealed combustion," and they have an additional PVC pipe for bringing in the combustion air from outside (two holes). Still another variation is a "pipe within a pipe," used for the highest efficiency, which uses a single pipe for both jobs.

Hope this helps!

--KenRad

 

[krb]

The "pipe in a pipe" mentioned is called a concentric vent. These types of furnaces use no inside air like other furnaces. Depending on the location of your furnace, this is a benefit because it reduces the amount of air infiltration to replace air drawn into the furnace for combustion. The vent should be insulated which will eliminate condensation. When properly installed, these furnaces are much more economical to operate. You can look at the websites for Trane and Carrier for more information about these furnaces. Also, any basic HVAC book should have information on how to calculate the cost savings.

 

[gearguru]

Thanks, gentlemen!
Now I am smarter.
Have a nice weekend!
gearguru

 

[PacificSteve]

Just one last thought....in crunching the numbers, it is important to know how each manufacturer calculates the number, based on HHV or LHV. Using LHV will give a higher efficiency number, but is not really honest. Probably most manufacturers state their efficiency based upon LHV, so maybe they are all being misleading to the same degree.

PacificSteve